Heliostat Control Scheme Using Cameras

ABSTRACT

A heliostat control system includes a heliostat having a reflective surface and at least one reflective element, the reflective surface having a different radius of curvature than the at least one reflective element, a receiver configured to receive sunlight reflected from the reflective surface, and a camera configured to receive sunlight reflected from the at least one reflective element and to generate an image including pixels having a brightness dependent on an orientation of the reflective surface.

TECHNICAL FIELD

The present disclosure relates generally to controlling heliostats using cameras.

BACKGROUND

A concentrating solar power system generally includes a number of heliostats configured to reflect light into a central receiver. The resulting heat can then be converted into power. Use of heliostats as a source of solar energy often requires receiver temperatures of nearly 1000° C., which in turn requires sunlight to be reflected from the heliostats into the receiver at high concentrations.

SUMMARY

In one aspect, a heliostat control system includes a heliostat having a reflective surface and at least one reflective element, the reflective surface having a different radius of curvature than the at least one reflective element, a receiver configured to receive sunlight reflected from the reflective surface, and a camera configured to receive sunlight reflected from the at least one reflective element and to generate an image including pixels having a brightness dependent on an orientation of the reflective surface.

Implementations can include one or more of the following. The at least one reflective element may have a greater radius of curvature than the reflective surface. The at least one reflective element may be circular in shape. The at least one reflective element may be convex.

The at least one reflective element may be on the reflective surface. The at least one reflective element may be located proximate to a center of the reflective surface. There may be a plurality of reflective elements. At least some of the reflective elements may be located proximate to corners of the reflective surface. The at least one reflective element may include a first portion and a second portion, the first portion having a lower reflectivity than the second portion. The first portion of the at least one reflective element may be closer to a center of the reflective surface than the second portion. The at least one reflective element may be generally tubular in shape. The at least one reflective element may be bowed outward from the reflective surface. There may be a plurality of heliostats, and the receiver may be configured to receive sunlight reflected from a reflective surface of each of the plurality of heliostats, and the image may include pixels having a brightness depending on an orientation of each of the reflective surfaces.

A controller may be configured to receive the image from the camera and calculate an error in the orientation based upon the image. The controller may be configured to associate pixels of the image with the at least one reflective element. There may be plurality of reflective elements, and the controller may be configured to calculate the error by determining whether a brightness of pixels associated with one reflective element differs substantially from a brightness of pixels associated with a different reflective element. The controller may be configured to send a signal to change the orientation of the mirror based upon the determined error. There may be a plurality of reflective elements, and the controller may be configured to repeat the steps of receiving an image, calculating an error, and sending a signal until a brightness of pixels associated with one reflective element is substantially equivalent to a brightness of pixels associated with each of the other reflective elements. The controller may be configured to associate a portion of the image with the heliostat.

A plurality of cameras may be positioned on different sides of the receiver, and a controller may be configured to receive images from the cameras. The controller may be configured to generate an error signal from a comparison of intensity values from the images from the cameras. The controller may be configured to cause the heliostat to point toward a location off-center of the receiver. The controller may be configured to subtract an intensity value from a first image from a first camera of the plurality of cameras from an intensity value from a second image from a second camera of the plurality of cameras to generate an error signal, and the controller may be configured to control orientation of the heliostat so that the error signal reaches a non-zero target value.

In another aspect, a method of heliostat control includes receiving sunlight in a receiver, the sunlight received in the receiver reflected from a reflective surface of a heliostat, receiving sunlight in a camera, the sunlight received in the camera reflected from a at least one reflective element of a heliostat, the at least one reflective element having a different radius of curvature than the reflective surface, generating an image from the sunlight reflected into the camera, and determining an error in an orientation of the reflective surface based upon the image.

Implementations can include one or more of the following. The sunlight may be reflected from a plurality of reflective surfaces, and each reflective surface may have a corresponding heliostat, and determining may include determining an error in an orientation of each of the reflective surfaces. Pixels of the image may be associated with the at least one reflective element. There may be plurality of reflective elements, and calculating the error may include determining whether a brightness of the pixels associated with one reflective element differs substantially from a brightness of pixels associated with a different reflective element. A signal may be sent to change the orientation of the reflective surface based upon the determined error. There may be a plurality of reflective elements, and the steps of generating an image, determining an error, and sending a signal may be repeated until a brightness of pixels associated with one reflective element is substantially equivalent to a brightness of pixels associated with each of the other reflective elements. A portion of the image may be associated with the heliostat. The camera may be cooled with a cooling system. Determining an error may include comparing images generated from a plurality of cameras. Determining an error may include comparing the image with an expected image. The heliostat can be controlled to point toward a off-center location of the receiver. Controlling the heliostat can include subtracting an intensity value from a first image from a first camera of a plurality of cameras from an intensity value from a second image from a second camera of the plurality of cameras to generate an error signal, and adjusting orientation of the heliostat so that the error signal reaches a non-zero target value.

In another aspect, a method of heliostat control may include receiving sunlight in a receiver, the sunlight received in the receiver reflected from a reflective surface on a heliostat, receiving sunlight in a camera, the sunlight received in the camera reflected from at least one reflective element of a heliostat, oscillating the reflective element at a frequency, generating first image from the sunlight reflected into the camera, and assigning a portion of the image to the heliostat by detecting the frequency of oscillation in the first image.

Implementations can include one or more of the following. A second image may be generated from the camera, the assigned portion may be located in the second image, and an error in an orientation of the reflective surface may be determined based upon the assigned portion. A signal may be sent to change the orientation of the reflective surface. Sunlight may be received in the receiver from reflective surfaces of a plurality of heliostats. The sunlight may be received in the camera reflected from reflective elements of a plurality of heliostats. Each of the plurality of heliostats may be oscillated at different frequencies, and different portions of the image may be assigning to each of the plurality of heliostats by identifying the different frequencies of oscillation in the first image. Certain implementations may have one or more of the following advantages. Using cameras to detect the positioning of a heliostat can provide more precise positioning, be less expensive, require fewer components, and require less frequent maintenance and calibration than other tracking mechanisms, such as placing a sensor on each heliostat. Detecting the positioning of the heliostats allows the heliostats to be adjusted to provided higher concentration of the sunlight in the receiver. Higher concentration of sunlight in the receiver provides a higher quality or temperature of heat for the production of solar power.

By including reflective elements having a different radius of curvature than the heliostat mirror, a more accurate determination of an error in the orientation of the heliostat can be made. More accurately determining an orientation error allows the heliostat orientation to be more precisely adjusted. Adjusting the heliostat orientation precisely allows the receiver to have a higher concentration of sunlight for a greater fraction of time.

Rotating a feature on a heliostat at a known frequency provides an accurate determination of which portions of an image of the heliostat field correspond to a particular heliostat. Accurate assignment of portions of the heliostat field image allows for the adjustment of the proper heliostat. Adjusting the proper heliostat ensure that the receiver will have a higher concentration of sunlight for a greater fraction of the time.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a heliostat control system including cameras located inside the receiver.

FIG. 2 is a schematic diagram of a camera system having a view port located inside the receiver and a camera located outside of the receiver.

FIG. 3 is a schematic diagram of a heliostat control system including cameras located outside of the receiver.

FIG. 4 is a schematic diagram of a heliostat having reflective elements near the corners of the mirror.

FIG. 4A is a schematic diagram of a heliostat mirror having reflective elements.

FIG. 4B is a schematic cross-section of a reflective element.

FIG. 5 is a flow chart of an exemplary method for adjusting the orientation of a heliostat.

FIG. 6 is a schematic diagram of a heliostat having bowed reflective elements along the edges of the heliostat.

FIG. 7 is a schematic diagram of a heliostat control system including cameras located outside of the receiver and reflective elements near the corners of the mirror.

FIG. 8 is a graph of an error signal as a function of heliostat position.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

When heliostats are used as a source of heat (which can in turn be used as a source of power), the concentration of sunlight reflected into the heat-collecting receiver can be lower than the theoretical ideal due to misalignment of the heliostat mirrors caused by errors such as difficulties detecting the orientation of the mirrors or relative position of the receiver, deformation of the mirror, or movement of the heliostat or receiver by natural causes. Using a camera to generate an image including pixels having a brightness dependent on the orientation of the heliostat mirrors allows sunlight to be more accurately reflected into the receiver. Moreover, having reflective elements on the mirror of the heliostat that have a different radius of curvature from the heliostat allows for more precise identification of errors in the heliostat orientation.

Referring to FIG. 1, a heliostat control system 500, e.g., for a solar power plant, includes a field of heliostats 100, which can include up to hundreds or thousands of heliostats (only two heliostats 100 a, 100 b are shown in FIG. 1). Each heliostat 100 includes a mirror 160 having a reflective surface 165 on the face of the mirror 160 closest to the sun 300. The reflective surface 165 can be flat or curved for better optical performance. Moreover, the reflective surface 165 can be in the shape of a quadrilateral, e.g., a square, or it can have rounded edges, e.g., be circular. The mirror 160 can rest on a foundation 110, which can be partially below ground.

An actuation system 132 is configured to move the heliostat mirror 160. The actuation system 132 can include multiple motors, such as a motor 120 to move the heliostat 100 in the azimuth direction using a motor shaft 130, and a motor 125 to adjust the altitude, i.e., angle of elevation, of the heliostat using a motor shaft 135. The actuation system 132 further includes control circuitry, e.g., a programmed microprocessor 172 and a transceiver 190 to receive commands directing the movement of the mirror 160. Wires 195 can electrically connect the transceiver 190 with the motors 120, 125, and the microprocessor 172 can convert the commands received by transceiver 190 into voltage signals on the wires 195 to control the motors 120, 125 and thus to control the orientation of the mirror. In other implementations, the actuation system can include hydraulic, pneumatic, cable and pulley, ballasted, or ball and socket mechanisms to move the heliostat mirror in the azimuth direction and/or to adjust the altitude.

The heliostat control system 500 further includes a tower system 200. The tower system includes a receiver 230 to receive sunlight and a camera 250, which can optionally include a filtering element 255 to reduce the intensity of the sunlight, and an optical element 259 (see FIG. 2) to expand, contract, or condition the sunlight as necessary prior to entry of the beam into the camera. The receiver 230 can be shaped to receive concentrated sunlight, such as be circular in shape. The region in which the receiver is located can be called the “receiver volume” or the “hot region” of the tower system 200. The receiver 230 can be located inside a housing 220 and can sit on top of a foundation structure 210, which can be partially below ground.

The heliostat control system 500 further includes a programmed microprocessor or computer 290 to receive image data from the cameras 250, to compute the movement of any heliostat mirrors 160 necessary to keep the heliostat oriented to reflect light to the receiver 230, and to send commands to the transceivers 190 of the heliostats. The computer 290 can be part of the tower system 200, as shown in FIG. 1. The computer 290 includes its own transceiver 174 to communicate with the transceivers 190 of the heliostats. The connection between the transceivers 190, 174 can be wired or wireless.

In operation, sunlight rays 320, 310 from the sun 300 can strike the reflective surface 165 of the heliostat mirrors 160. The reflective surface 165 can then reflect rays 321, 311 towards the receiver 230. The reflected rays 321, 311, in addition to rays reflected from other heliostats in the field, can heat the receiver 230 to temperatures of between 900° C. and 1200° C., such as between 950° C. and 1150° C. The heat can be used to drive various heat engines to produce power. For example, the heat can be used to warm cold air, which can then be expanded through a turbine engine which turns a generator shaft, which creates power. The more concentrated the sunlight is in the receiver 230, the higher the temperature of the receiver 230, and the more efficient the power generation of the system 500 can be.

In order to maximize the concentration of rays on the receiver 230, the normal vector of the reflective surface 165 must bisect the angle between the rays 310, 320 from the sun and the rays reflected towards the center of the receiver 230. Thus, as the sun 300 moves across the sky, the orientation of the reflective surface 165 of the mirrors 160 must be adjusted to ensure that the reflected rays are hitting the receiver 230 without causing too much spillage, i.e., causing too many rays to be reflected outside of the receiver 230.

The camera 255 mounted on the receiver 230 can be used to determine whether a particular mirror 160 is oriented to reflect substantially the maximum amount of light into the receiver, i.e., to orient the reflective surface 165 such that its normal vector substantially bisects the angle between the sun and the receiver. When rays from the heliostats 100 are reflected into the receiver 230, and correspondingly to the camera 255, the camera 255 observes and produces an image. The image produced by the camera 250 can include different portions, e.g., groups of pixels, having a brightness dependent on the orientation of the different heliostats. As a result, as discussed below, the image can be used to determine an error in the orientation of the mirrors.

Initially, the computer 290 will establish communication with the heliostats 100 in the field. In brief, the computer 290 includes a transceiver and maintains a communication network, e.g., a wired or wireless LAN. When the heliostat is installed and connected to the communication network, it can be assigned a network address, e.g., an Internet Protocol (IP) address, in a conventional manner. Thereafter the computer 290 can have a database including a unique identifier for each heliostat in the field and information regarding how to communicate, e.g., the network address, with each heliostat.

Since the position of the heliostat, e.g., the position in the image produced by the camera, is not necessarily known by the computer 290, a calibration step can be performed prior to determining the actual error in orientation of a heliostat. During calibration, the assignment of a particular heliostat to a set of pixels in the camera's imaging array, can be determined. For example, during the calibration step, the camera 250 can observe and produce an image of the heliostat field. Portions of the image, or groups of pixels, can be associated with a particular heliostat.

The calibration step can be automated. In some implementations, the computer 290 can send a signal to the microprocessor 172 of a newly installed heliostat, e.g., a heliostat which has established communication on the network but has not been calibrated yet, to cause the heliostat to oscillate at a predefined frequency, i.e., a frequency stored in the computer 290. The computer 290 can then find, e.g., using conventional video processing techniques, that frequency of blinking in the image in order to identify the portion of the pixel array of the image that corresponds to the heliostat. The portion of the image corresponding to the heliostat is then stored by the computer 290. In another implementation, a spinning or rotating feature can be placed on or proximate to the heliostat. The RPM of the spin can match the flicker frequency of the image such that the computer 290 can identify the heliostat in the pixel array of the image. The rotation or spinning can be performed, for example, with a motor or with wind. If rotated or spun by the wind, an optical encoding method can be used to identify the rate of spin, and the computer 290 can then match the frequency to a particular heliostat. Proceeding through each of the heliostats sequentially, each heliostat in an entire field of heliostats may be identified. Such a mechanism is important for unique identification of each heliostat. Alternatively, the computer 290 can maintain a database that associates different frequencies different heliostats, cause the multiple heliostats to oscillate simultaneously at the different frequencies, and associate different portions of the image with the different heliostats essentially simultaneously by identifying the different frequencies of oscillation.

In another implementation, two or more spinning or rotating features can be placed on or proximate to the heliostat. Including two or more spinning or rotating features on a particular heliostat helps ensure that the heliostat will have a corresponding oscillation frequency that is unique from other heliostats in the field. For example, if there are five unique frequencies of oscillation, and there are two spinning or rotating mechanisms on each heliostat, then there can be ten unique combinations of identification signals. Further, if the position of the spinning or rotating mechanism can be resolved in the image, then there are five to the power of two, or twenty-five, unique ordered combinations of unique signals. In other implementations, a rotating or spinning feature can be oscillated at increasing or decreasing frequencies or can be oscillated for a unique duration period to generate a unique signal from each heliostat. Using features that can produce unique signal can be useful for initial calibration of the heliostats. For example, each heliostat can be identified sequentially, as in the implementation described above. Alternatively, because each heliostat has a unique oscillation frequency, multiple heliostats can be identified simultaneously from a single set of images. Further, having unique identification signals for each heliostat can be useful for identification of heliostats during use, e.g., to identify a particular heliostat that may need to be repaired or adjusted. In general, if a heliostat that is flickering at a specific frequency that no other heliostat flickers at, then the heliostat has a unique flicker frequency and can be identified with video processing.

In some implementations, the physical position and approximate size of the heliostat relative to the receiver is stored by the computer 290, e.g., manually entered by an operator into the computer 290 and/or into the microprocessor 172 and transmitted to the computer 290. Assuming that the orientation of the camera 250 is stored by the computer 290, the computer 290 can be configured to calculate the portion of the image corresponding to the heliostat from this positional information.

In some implementations, calibration step can be performed manually. For example, the image from the camera 250 can be displayed to a user, and the user can use an input device to indicate the portions of the image associated with the heliostat, e.g., by clicking with a mouse on the location in the image.

In any event, once the calibration is performed, the database in the computer 290 can associate each of the heliostats in the field with the portions of the image corresponding to each of those heliostats.

Because there are generally multiple heliostats in a field, it may be advantageous to be able to detect when a heliostat is physically moved, or when a particular heliostat is added or removed from the field. Thus, in some implementations, the computer periodically compares new images with the original calibration images to look for changes. New bright spots that were not present in a previous image may indicate new heliostats, or that a heliostat has been moved. The system can automatically detect the positions of all of the heliostats in the field and recalibrate as necessary.

After the computer 290 has assigned a portion of an image to each heliostat in a field, the system 500 can determine an error in an orientation of the mirrors (i.e. to determine whether the mirror is oriented to reflect a maximum amount of light into the receiver 230) and subsequently to change the orientation of the mirrors 160 such that they reflect substantially the maximum amount of light into the receiver 230. In one implementation, the computer 290 can try to maximize the sunlight seen by the camera 250 from each heliostat. If a portion of the image assigned to a particular heliostat does not include a bright spot, or includes a spot that is not as bright as expected, the computer 290 can determine that the mirror 160 for that heliostat is not oriented accurately. For example, in FIG. 1, ray 321 is not hitting the center of the receiver 230. As a result, the image produced by the camera 250 will not be as bright in the portion of the image corresponding to heliostat 100 b as expected. In another implementation, discussed further below with respect to FIGS. 4-6, the computer 290 can try to equalize the sunlight reflected from multiple reflective elements on or proximate to the mirror, each reflective element having a different radius of curvature than the heliostat mirror.

The computer 290 can send a command to motors on a particular heliostat, such as heliostat 100 b in FIG. 1, to direct the heliostat to move its mirror 160 accordingly. For example, the computer 290 can send a command, such as through a wireless or wired signal, through a transceiver 190 on the heliostat, which can in turn send a signal through wires 296 (or optionally can send a wireless signal) to the motors 120, 125 on the heliostat. The motors can in turn adjust the mirrors 160 as directed.

For example, the motor 120 might first be commanded by the computer 290 to move a mirror 160 of a heliostat in a particular direction along the azimuth. If the brightness for the portion of the image assigned to that particular heliostat increases, then the computer 290 can command the motor 120 to continue to move the mirror 160 in that direction. In contrast, if the brightness decreases, then the computer 290 can command the motor 120 to move the mirror 160 in the opposite direction. These adjustment steps can then be repeated for elevating the mirror with motor 125.

The system 500 can also use additional factors to optimize the movement of the heliostats. For example, the computer 290 can take into account celestial data and data of previous days' heliostat orientation paths. Further, the computer 290 can observe the image of the sun and use its distinct features, such as solar flares, to determine which direction to move.

As the receiver 220 is a hot, harsh environment, the camera 250 may need to be protected from the heat. In some implementations, a cooling system can be used to protect the camera. For example, a jacket of coolant surrounding the camera can be connected to an external liquid coolant circulation system. Filters in front of the camera can allow only light at particular wavelengths to enter the camera while keeping the majority of the thermal energy away from the camera.

In some implementations, shown in FIG. 2, a camera system 600 can be used to protect the cameras 250 from the heat. The camera system 600 includes a view port 253 constructed of materials that withstand intense heat, such as ceramics, minerals, and metal alloys. In some implementations, the materials are cooled in order to increase their resistance to heat. The view port 253 can extend through the receiver 230, such as through the center of the receiver or just off the center of the receiver, or it can extend just outside of the receiver. The view port 253 can include an aperture that allows in only small amounts of light to pass, which can limit the amount of damaging thermal energy that enters the camera and can help sharpen the image. The camera system 600 can further include a filter 255, optics 259, and a camera 250. A reflecting mirror 257 can be positioned on a base 258 between the view port 253 and the camera 250. The reflecting mirror can be tilted as an angle between 0° and 90°, such as between 30° and 60°, for example 45°. Although not shown, a shading layer can extend outward from the receiver to keep spilled sunlight from hitting and damaging the camera 250 and related components. This shading structure could be made of reflective material, ceramic material, refractory material, or actively cooled material. Further, in some implementations, the optical path can be routed from the viewport to cameras located behind the receiver itself.

In operation, sunlight 351 reflected from heliostats 100 (not shown in FIG. 2) can travel through the view port 253, such as through an aperture in the view port. The sunlight can further travel through the filter 255 to reduce the intensity of the sunlight to avoid damaging of the camera 250 and other components such as mirror 257 or optics 259. Sunlight can then reflect off of the deflecting mirror 257 in order to move the sunlight from the region of the heat engine Finally, the sunlight can travel through optics 259 to expand, contract, or condition the sunlight as necessary to get the best accuracy, prior to entry of the sunlight into the camera. As in the implementation described above, the camera can then produce an image including pixels having a brightness dependent on the orientation of the mirrors 160 of heliostats in the field. Such a configuration allows the rays to be sensed close to, or at the border of, the receiver 230 while keeping the electronic components of the camera 250 away from the hot environment of the receiver 230.

In other implementations, shown in FIG. 3, a heliostat control system 700 includes multiple cameras 250 placed outside of the receiver 230. Multiple cameras, e.g. four cameras, can be spaced around the receiver, e.g., above and below and to either side of the receiver 230. The cameras 250 can be placed in proximity to the receiver where there is likely to be more spillage of sunlight, i.e., rays missing the receiver. For example, if the reflecting element 165 is rectangular in shape, and the receiver is circular in shape, the cameras can be placed around the corners of the receiver where there is likely to be more spillage.

In operation, each camera 250 can produce a separate image of the same portion of the heliostat field. As discussed above, the computer 290 can determine whether a particular heliostat is misaligned by analyzing the image data for that particular heliostat. In this implementation, however, because the cameras 250 are not centered in the receiver 230, the amount of sunlight seen by the camera from each heliostat 100 or reflective element (as discussed with respect to FIGS. 4-6) of the heliostat is not maximized. Rather, the image can be compared to the expected brightness, whether more or less bright, and the heliostats adjusted accordingly.

Additionally, when multiple images are produced from different cameras, the resulting images can be compared to one another. If the brightness is different from one camera to another in the portion of the image that corresponds to a particular heliostat or reflective elements of a heliostat, then the computer 290 can determine that the mirror of that particular heliostat is misaligned. For example, as shown in FIG. 3, the ray 321 from heliostat 100 b to the receiver 230 is not hitting the center of the receiver. As a result, the camera 250 a, to which the ray 321 is closer, will see a brighter spot in the portion of the image corresponding to the heliostat 100 b than will camera 250 b.

The computer 290 can then command the motors of a misaligned heliostat to adjust the mirror 160. As in the implementation described above, the motors 120, 125 can first be commanded to move a particular heliostat in a particular direction. In this implementation, the mirror 160 can be moved by the motors until the mirrors see approximately equal brightnesses from the heliostat. For example, in FIG. 3, mirror 160 of heliostat 100 b can be moved such that the reflected ray 321 moves further away from camera 250 a and closer to camera 250 b. If the brightnesses of the portions of the images corresponding to heliostat 100 b become equal for both cameras 250 a and 250 b, then the computer 290 can command the motors to stop moving the heliostat. In contrast, if camera 250 b begins to see more light from 100 b than camera 251, the computer 290 can command the mirror 160 to move back in the opposite direction.

When cameras 250 are placed outside of the receiver 230, as shown in FIG. 3, the cameras need not receive spillage from the rays all of the time in order to be effective. Rather, the system can be configured to be triggered only when the reflected sunlight drifts out of the receiver and onto one of the cameras 250.

In some implementations, examples of which are shown in FIGS. 4 and 6, reflective elements 402 can be placed on or proximate to the mirror 160. If the reflective elements are not on the mirror 160 itself, they can be supported on the same support structure as the mirror 160. Thus, the reflective elements remain stationary relative to the mirror, i.e., the reflective elements will move with the mirror when the mirror is actuated by the motors 120, 125. The reflective elements can have a different radius of curvature than the mirror 160 such that at least some of the sunlight hitting the reflective elements will reflect in a different manner than sunlight hitting the mirror 160, i.e., will reflect sunlight even when the heliostat mirror does not.

In one implementation, shown in FIGS. 4, 4A, and 4B, the reflective elements can be approximately hemispherical. Referring to FIG. 4B, the mirror 160 can be approximately planar or flat. A mirror 160 can have a focal point ranging from 1 m to infinity, such as between 10 m and 200 m. In contrast, the reflective elements 402 can be convex. The reflective elements 402 can have a diameter between, for example, 1 cm to 10 cm. Further, the radius of curvature of the reflective elements 402 can range from 1 cm to 1 m. As shown in FIGS. 4 and 4A, the reflective elements 402 can be placed proximate to the corners of the mirror 160. Alternatively, or in addition, the reflective elements 402 can be placed in other areas, such as along the edges or near the center of the mirror 160. Although FIGS. 4 and 4A show four reflective elements 402, there can be a different number of reflective elements, such as 2, 3, or 8 reflective elements.

Each reflective element 402 can have a masked region 404 having a lower reflectivity than the unmasked portion. The masked region 404 can be oriented towards a center of the mirror 160, and can cover a portion of the reflective element, for example ⅛ to ⅓, such as ¼, of the surface area of the reflective element. For example, on the hemispherical reflective element, the masked region can be a wedge-shape. In an alternate implementation (not shown), the masked portions 404 can be oriented away from a center of the mirror 160 and can cover a larger portion of the reflective element, for example ⅔ to ⅞, such as ¾, of the surface of the reflective element. In this implementation, on the hemispherical reflective element, the unmasked region can be a wedge-shape.

In operation, using system 500 or 700, the computer 290 can associate the pixel or groups of pixels in the camera's imaging array with each reflective element of a particular heliostat. Because sunlight is reflected from the mirror 160 and reflective elements 402 differently, the reflective elements will show up as spots of higher or lower intensity (compared to the mirror 160) within or adjacent to the portion of the image that corresponds to the mirror 160 of the heliostat. The computer 290 can thus pick out the portions of the image with differing intensities relative to the rest of the mirror and associate those portions with the reflective elements.

The reflective elements 402 can then be used to determine an error in orientation of the mirror 160. Referring to FIG. 5, sunlight can be reflected from the reflective elements 402 into the camera 290 (step 502). At the same time, sunlight reflected from the mirror 160 can be reflected into the receiver 230 (step 504). An image can be generated from the sunlight reflected into the camera (step 506).

The reflective elements 402 can be configured such that each reflective element 402 reflects substantially an equal amount of sunlight when the mirror 160 is oriented correctly, i.e., with a normal bisecting the angle between the sun and the receiver (step 512). For example, in the configuration of FIG. 4, there are four reflective elements 402, each one having ¼ of the reflective surface masked off, and each masked portion 404 oriented towards the center. Because the masked portions 404 of the reflective elements 402 are oriented towards the center of the mirror 160, and assuming that the reflectivity of the masked portions is the same, the portions of the image assigned to the reflective elements 402 should have a lower intensity, (e.g. no intensity if the camera is directly between the sun and the heliostat), relative to the rest of the mirror 160 because a portion of the sun would be blocked by the masked portions. Moreover, each reflective element should have an intensity nearly equivalent to the other reflective elements when the mirror 160 is aligned to perfectly bisect the angle between the rays from the sun and the rays reflected towards the receiver. Small errors in having an exactly equal intensity could occur, for example, if the camera is not directly in the center of the receiver. Thus, the reflective elements must only reflect “nearly” or “substantially” equal light when the mirror is oriented correctly. Similar results would occur for any number of reflective elements provided that there were x reflective elements equally spaced around the mirror 160 with 1/x of the element masked off and oriented towards the center of the mirror.

Furthermore, if there were x reflective elements equally spaced around the mirror 160 with (x−1)/x of the element masked off and oriented away from a center of the mirror, the computer 290 could determine that the mirror 160 was oriented correctly if all of the reflective elements generated light at nearly equal intensity. In such a setup, if the pixels associated with each reflective element show similar brightness (i.e. if all of them reflect light), the computer 290 can determine that the heliostat mirror 160 is oriented correctly (step 508).

On the other hand, if the brightness of a portion of the image assigned to one reflective element differs substantially from a portion of the image assigned to another reflective element, then the computer 290 can determine that there is an error in the orientation of the mirror 160 (step 508). For example, as shown in FIG. 4A, only the reflective elements 402 in the upper right, upper left, and lower left corners of the mirror 160 are reflecting rays into the receiver 230. The reflective element 402 a in the lower right hand corner is reflecting less sunlight (e.g., no sunlight) than the other reflective elements 402 because the heliostat mirror 160 is oriented such that the sunlight that would ordinarily reflect to the cameras is primarily hitting the masked portion 404 of that reflective element. As a result, the light intensity of the pixels in portion of the image associated with that reflective element will be lower than the light intensity in the portions of the image associated with the other reflective elements, and the computer 290 can determine that there is an error in orientation of the mirror 160.

On the other hand, for the situation where a camera, such as cameras 250 a or 250 b, is observing the heliostat field from outside of the receiver, it is possible that the reflective elements 402 will bounce more light than their adjacent ones on the same heliostat when the heliostat is properly oriented.

To address this situation, multiple cameras, such as cameras 250 a and 250 b, can be used to observe the heliostat field from outside of the receiver. In this case, the heliostat can be considered correctly oriented if the different cameras receive similar intensities from different groups of reflective elements. More particularly, each camera can be associated with a group of reflective elements that is on the same side of the mirror as the camera is on the receiver. If the first group of reflective elements associated with a first camera and the second group of reflective elements associated with a second camera have similar intensities, then the heliostat can be considered correctly oriented.

For example, referring to FIG. 7, For example, referring to FIG. 7, camera 250 a can see two of the reflective elements, e.g., elements 402 a and 402 b as hot (e.g., the Easterly ones), and the other two reflective elements 402 c and 402 d (e.g., the Westerly ones) as cold. Conversely, camera 250 b sees the reflective elements 402 a and 402 b as cold, but the sees the reflective elements 402 c and 402 d as hot. Therefore, the heliostat is balanced and pointing the sun into the receiver. More particularly, assuming that the cameras 250 a and 250 b are spaced equidistant and on opposite sides of the receiver 230, reflective elements 402 a and 402 b are associated with camera 250 a, and reflective elements 402 c and 402 d are associated with camera 250 b. If the intensities from reflective elements 402 a and 402 b as measured by camera 250 a is equal to the intensities from reflective elements 402 c and 402 d as measured by camera 250 b, then the heliostat is correctly oriented.

Moreover, if there is an error in orientation, the computer 290 can use the reflective elements to determine a direction in which to reorient the mirror 160. In the implementation in which there are x reflective elements equally spaced around the mirror 160 with a portion of the element oriented towards the center of the mirror being masked off (as shown in FIG. 4), a dark spot (or less intense spot) associated with one of the reflective elements relative to the other reflective elements suggests that the mirror needs to be rotated about an axis 436 that is perpendicular to an axis 438 through the center of the reflective element and bisecting the reflective element along the outer radius. Further, the reflective element 402 showing lesser intensity needs to be rotated towards the receiver. For example, in the configuration of FIG. 4A, the computer 290 can determine that the mirror 160 needs to be rotated such that the reflective element 402 in the lower right hand corner is oriented slightly more towards the sun. The computer 290 can then send a signal to reorient the mirror 160 (step 510). Thus, referring to FIG. 4A, the mirror 160 would be rotated such that the reflective element 402 in the lower right hand corner comes out of the page more, while the reflective element 402 in the upper left hand corner is moved into the page more. Conversely, if the reflective element 402 shows a higher intensity, then the mirror needs to be rotated about the axis 436 to move the reflective element 420 rotated away from the receiver. The process can be iterated with successively smaller step until the pixels associated with each reflective element show nearly equal brightness.

On the other hand, if there were x reflective elements equally spaced around the mirror 160 with a portion of the element oriented away from a center of the mirror masked off, then a reflective element having a different (higher) intensity than the rest would need to be rotated about the same axis perpendicular to the axis bisecting the reflective element, but would need to be rotated away from the receiver. Conversely, if the reflective element 402 shows a lower intensity than the others, then the mirror needs to be rotated about the axis 436 to move the reflective element 420 toward the receiver.

More generally, the computer 290 can use the relative intensities of the reflective elements to determine a direction in which to rotate the mirror to reduce angular difference between the normal vector of the reflective surface and the vector bisecting the vectors from the reflective surface to the sun and the center of the receiver 230. Alternative implementations of the reflective elements are possible, provided that a radius of curvature of the reflective elements is different from the radius of curvature of the mirror. For example, in another implementation, shown in FIG. 6, reflective elements 602 can be placed along the edges of the mirror 160. The reflective elements 602 can be approximately tubular, and the length of each tube can run approximately parallel with a side of the mirror 160. Similar to the implementation described above, the mirror 160 can be approximately planar or flat, while the reflective elements 602 can be convex (i.e. bowed outwards). The center of each reflective element 602, i.e. the peak of each bowed portion, can be at approximately at center of a corresponding side of the mirror 160. Using system 500 or 700, pixels of the generated image can be assigned to each reflective element 602, in particular to the center of each reflective element 602. The computer can try to equalize an amount of sunlight seen by the camera from the center portions of each reflective element 602. For example, as shown in FIG. 6, if the mirror is not oriented correctly, sunlight from the sun 300 will not reflect from the center of each reflective element 602. In the configuration of FIG. 6, the computer 290 can determine that the mirror 160 is reflected too far to the right (because the light is being reflected from the right portions of the reflective elements 602). The mirror 160 can thus be readjusted, as discussed above, until the brightness from the center portions of each of the reflective elements 602 is nearly equal.

The sunlight reflected from the reflective elements can be treated as binary information. For example, if sun is reflected from a reflective element, it can be considered “on” or one, while if no sun is reflected from a reflective element, it can be considered “off” or zero. Alternatively, the sunlight reflected from the reflective elements can be treated as a quantified relative value.

In some implementations, multiple cameras can be placed in the receiver. In some implementations, multiple view ports can be placed in the receiver, each view port connected to a different camera. In some implementations, multiple cameras can be mounted at a distance from the receiver, and optical paths can be used that project the image to the cameras.

In some implementations the projected image from the field can be incident on the front side of a semitransparent screen. The cameras can observe the backside of this screen, where the intensity is drastically reduced. The screen could be made of glass, fabric, or thin ceramic.

In some implementations, the reflective elements can be faceted to more directly concentrate the sunlight on the receiver. In other implementations, the reflective elements can have a pyramidal or three-dimensional polygonal shape.

In some implementations, where the reflective elements are approximately tubular (as shown in FIG. 6), the reflective element can be flatter near the middle and parallel to the mirror surface, and more sloped near the ends (e.g. parabolic or hyperbolic) and thus not parallel to the mirror surface). As a result, a reflected sunlight spot on the tube can move at the same rate as the mirror 160 in the flat parts (i.e. when alignment is close), but at a different rate from the mirror near the ends (i.e. when alignment is bad). This relative difference can be used to determine the orientation of the mirror.

In controlling the speed of the heliostat, if the error is large, the heliostat can be moved at a high velocity. As the error gets small and the heliostat approaches the desired position, the speed can be decreased. By shaping the reflective bar differently, the error signal automatically changes more significantly when the error is large, but slower when the geometries are just about right. So given a constant velocity motor or mechanism, the error signal derived from the reflection, changes more or less rapidly.

In some implementations, an individual heliostat can be controlled to direct reflected light slightly off the center of the receiver, and a group of heliostats can be controlled to direct reflected light slightly at different locations (at least some of which are at different positions off the center of the receiver). By directing the reflected light at slightly different locations on the receiver, flux from the heliostats can be smoothed out on the receiver.

Referring to FIGS. 7 and 8, one technique to accomplish pointing the heliostats off-center of the receiver is to uses light spillage as an error signal. For example, the intensity signal from the portion of the image from one camera, e.g., camera 250 b, corresponding to the mirror 160 of the heliostat 160, can be subtracted from the intensity signal from the portion of the image corresponding to the mirror 160 of the heliostat 160 from the camera on the opposite side of the center of the receiver, e.g., camera 250 a, to generate an error signal which depends on the angle of the heliostat. As shown in FIG. 8, assuming that the two cameras are equidistant from the center of the receiver, the error signal 800 when the beam from the heliostat is pointing to the center of the receiver, the cameras should receive equal light spillage, and the error signal should be 0 (indicated at line 802). As the beam from the heliostat moves off center in one direction, e.g., toward camera 250 a, the error signal increases, until it reaches a maximum 804 when the beam is pointed directly at the camera 250 a. Conversely, as the beam from the heliostat moves off center in the other direction, e.g., toward camera 250 b, the error signal decreases, until it reaches a minimum 806 when the beam is pointed directly at the camera 250 a.

Using the above system, on-center control can be accomplished by adjusting the orientation of the heliostat mirror 160 so that the error signal goes to zero. In addition, a particular heliostat can be pointed slightly off center using this system by adjusting the orientation of the heliostat mirror so that the error signal goes to a small but non-zero target value. This non-zero value can be, e.g., up to 5% of the maximum value 802 or minimum value 806. The value of 5% above is merely an example; given a maximum desired spillage, e.g., a beam that is off-center by 10% of the width of the receiver, corresponding maximum and minimum target values can be determined empirically. By selecting different target values for at least some of the heliostats, these heliostats can be pointed at different locations of the receiver. While the discussion above and FIG. 7 features off-center control along a single axis, the heliostats can be oriented off-center in any direction by using signals from cameras positioned on multiple axes.

In some implementations, a reflective element is located proximate to the center of the heliostat. The reflective element can be configured to always reflect the sun when the mirror is aligned in the general direction of the sun, i.e. when the mirror is within 270° of the sun. For example, the reflective element can be generally hemispherical and have no masked sections. Therefore, provided that the mirror wasn't within the 90° window directly opposite the sun, the reflective element will always reflect light into the reflector and/or camera. The reflective element can therefore always allow the controller to find the center of the heliostat.

In some implementations, the reflective elements can include cross-hatches that appear different to the camera depending upon the angle.

Having individually identifiable reflective elements on or near the surface of the mirror can allow the controller to identify a particular orientation of a heliostat. Determining a particular orientation of the heliostat can allow for rotation of the heliostats to provide a closed-loop heliostat control system that ensures that sunlight is reflected from each heliostat into the desired receiving location. Given the available speed of image processing, errors in the heliostat reflection can be controlled on a real-time, or near-real time basis. Such a system allows concentrated sunlight to enter the receivers for a large fraction of the day in order to provide sufficiently high temperatures for the creation of solar power.

Particular embodiments have been described. Other embodiments are within the scope of the following claims. 

1. A heliostat control system, comprising: a heliostat having a reflective surface and at least one reflective element, the reflective surface having a different radius of curvature than the at least one reflective element; a receiver configured to receive sunlight reflected from the reflective surface; and a camera configured to receive sunlight reflected from the at least one reflective element and to generate an image including pixels having a brightness dependent on an orientation of the reflective surface.
 2. The heliostat control system of claim 1, wherein the at least one reflective element has a greater radius of curvature than the reflective surface.
 3. The heliostat control system of claim 1, wherein the at least one reflective element is circular in shape.
 4. The heliostat control system of claim 1, wherein the at least one reflective element is convex.
 5. The heliostat control system of claim 1, wherein the at least one reflective element is on the reflective surface.
 6. The heliostat control system of claim 5, wherein the at least one reflective element is located proximate to a center of the reflective surface.
 7. The heliostat control system of claim 5, wherein there are a plurality of reflective elements, and wherein at least some of the reflective elements are located proximate to corners of the reflective surface.
 8. The heliostat control system of claim 1, wherein the at least one reflective element includes a first portion and a second portion, the first portion having a lower reflectivity than the second portion.
 9. The heliostat control system of claim 8, wherein the first portion of the at least one reflective element is closer to a center of the reflective surface than the second portion.
 10. The heliostat control system of claim 1, wherein the at least one reflective element is generally tubular in shape.
 11. The heliostat control system of claim 10, wherein the at least one reflective element is bowed outward from the reflective surface.
 12. The heliostat control system of claim 1, wherein there are a plurality of heliostats, the receiver is configured to receive sunlight reflected from a reflective surface of each of the plurality of heliostats, and the image includes pixels having a brightness depending on an orientation of each of the reflective surfaces.
 13. The heliostat control system of claim 12, further comprising a controller configured to receive the image from the camera and calculate an error in the orientation based upon the image.
 14. The heliostat control system of claim 13, wherein the controller is further configured to associate pixels of the image with the at least one reflective element.
 15. The heliostat control system of claim 13, wherein there are a plurality of reflective elements, and wherein the controller is configured to calculate the error by determining whether a brightness of pixels associated with one reflective element differs substantially from a brightness of pixels associated with a different reflective element.
 16. The heliostat control system of claim 13, wherein the controller is further configured to send a signal to change the orientation of the mirror based upon the determined error.
 17. The heliostat control system of claim 16, wherein there are a plurality of reflective elements, and wherein the controller is configured to repeat the steps of receiving an image, calculating an error, and sending a signal until a brightness of pixels associated with one reflective element is substantially equivalent to a brightness of pixels associated with each of the other reflective elements.
 18. The heliostat control system of claim 13, wherein the controller is configured to associate a portion of the image with the heliostat.
 19. The heliostat control system of claim 1, further comprising a plurality of cameras positioned on different sides of the receiver and a controller configured to receive images from the cameras, and wherein the controller is configured to generate a error signal from a comparison of intensity values from the images from the cameras.
 20. The heliostat control system of claim 19, wherein the controller is configured to cause the heliostat to point toward a location off-center of the receiver.
 21. The heliostat control system of claim 20, wherein the controller is configured to subtract an intensity value from a first image from a first camera of the plurality of cameras from an intensity value from a second image from a second camera of the plurality of cameras to generate an error signal, and wherein the controller is configured to control orientation of the heliostat so that the error signal reaches a non-zero target value.
 22. A method of heliostat control, comprising: receiving sunlight in a receiver, the sunlight received in the receiver reflected from a reflective surface of a heliostat; receiving sunlight in a camera, the sunlight received in the camera reflected from at least one reflective element of a heliostat, the at least one reflective element having a different radius of curvature than the reflective surface; generating an image from the sunlight reflected into the camera; and determining an error in an orientation of the reflective surface based upon the image.
 23. The method of claim 22, wherein the sunlight is reflected from a plurality of reflective surfaces, each reflective surface having a corresponding heliostat, and wherein determining comprises determining an error in an orientation of each of the reflective surfaces.
 24. The method of claim 22, further comprising associated pixels of the image with the at least one reflective element.
 25. The method of claim 24, wherein there are a plurality of reflective elements, and wherein calculating the error includes determining whether a brightness of the pixels associated with one reflective element differs substantially from a brightness of pixels associated with a different reflective element.
 26. The method of claim 22, further comprising sending a signal to change the orientation of the reflective surface based upon the determined error.
 27. The method of claim 26, wherein there are a plurality of reflective elements, the method further comprising repeating the steps of generating an image, determining an error, and sending a signal until a brightness of pixels associated with one reflective element is substantially equivalent to a brightness of pixels associated with each of the other reflective elements.
 28. The method of claim 22, further comprising associating a portion of the image with the heliostat.
 29. The method of claim 22, further comprising cooling the camera with a cooling system.
 30. The method of claim 22, wherein determining an error comprises comparing images generated from a plurality of cameras.
 31. The method of claim 22, wherein determining an error comprises comparing the image with an expected image.
 32. The method of claim 22, further comprising controlling the heliostat to point toward a off-center location of the receiver.
 33. The method of claim 32, wherein controlling the heliostat includes subtracting an intensity value from a first image from a first camera of a plurality of cameras from an intensity value from a second image from a second camera of the plurality of cameras to generate an error signal, and adjusting orientation of the heliostat so that the error signal reaches a non-zero target value.
 34. A method of heliostat control, comprising: receiving sunlight in a receiver, the sunlight received in the receiver reflected from a reflective surface on a heliostat; receiving sunlight in a camera, the sunlight received in the camera reflected from at least one reflective element of a heliostat; oscillating the reflective element at a frequency; generating first image from the sunlight reflected into the camera; and assigning a portion of the image to the heliostat by detecting the frequency of oscillation in the first image.
 35. The method of claim 34, further comprising: generating a second image from the camera; locating in the second image the assigned portion; and determining an error in an orientation of the reflective surface based upon the assigned portion.
 36. The method of claim 35, further comprising sending a signal to change the orientation of the reflective surface.
 37. The method of claim 34, further comprising: receiving sunlight in the receiver, the sunlight received in the receiver reflected from reflective surfaces of a plurality of heliostats; receiving sunlight in the camera, the sunlight received in the camera reflected from reflective elements of a plurality of heliostats; oscillating each of the plurality of heliostats at different frequencies; assigning a different portions of the image to each of the plurality of heliostats by identifying the different frequencies of oscillation in the first image. 